Renal Function Assessment: The Strengths and Weaknesses of the Various Parameters
Section of Clinical Pathology, Faculty of Veterinary Science, University of Pretoria
Onderstepoort, Republic of South Africa
The routine laboratory parameters used to assess renal function in small animals have changed very little over the years. From time-to-time new tests are proposed but, almost invariably, the practitioner ends up using the same old profile in the long run. Changes that have taken place, less obvious to the clinician, are changes in the analytical methods applied to these "old" tests. This paper will briefly examine both the potential of "new" tests as well as the effect that changes in methodology imply for the clinician.
The classical profile, depending on the facilities available to the clinician, will include some or all of the following:
Serum urea and/or creatinine and/or other estimates of glomerular filtration rate (GFR)
Proteinuria and or electrophoresis for urinary protein
Urine specific gravity (SG) and/or water deprivation test
Electrolyte fractional clearance
Urine activity of brush border enzymes such as GGT: creatinine ratio
Tests estimating GFR
One of the most persistent problems with renal function assessment is the relative insensitivity of the traditional laboratory parameters of renal function, serum creatinine, one of the most reliable, only increasing after 75% of functional mass has been lost. Creatinine, although not significantly influenced by diet, is rendered even less sensitive in patients with a reduction in muscle mass as seen in emaciation and cachexia. Serum urea is markedly influenced by nitrogen balance (even short periods of anorexia reducing the load of urea on the kidneys) as well as ammonia loading. High protein intake as well as gastro-intestinal haemorrhage, lead to very significant increases in serum urea (especially in the lower-abnormal rage). Very severe tissue catabolism, as seen in metastatic neoplasia and therapeutic tumour lysis syndrome, may also cause significant ammonia and hence urea loading. Both the above routine indices of GFR have long been known to correlate poorly with true GFR, as assessed by Inulin clearance.
Since the mid-1990's a new test has been advocated that is less dependent on non-renal factors, Cystatin-C. This cysteine protease inhibitor, produced and released by all somatic cells, is reported to be less influenced by body condition and ammonia metabolism than are creatinine and urea respectively. It has taken until the end of 2001 for this test to make its appearance in the veterinary literature. Preliminary studies look promising. However, our own limited experience with this test has not provided overwhelming evidence of improved sensitivity, compared with creatinine. Even so, hopefully, when new research data become available, we may be in a position to determine whether Cystatin can overcome the problem of haemoglobinaemia and hyperbilirubinaemia, which interfere with creatinine determinations.
Non-creatinine chromogens-no longer an issue
Virtually every article and book chapter discussing creatinine's limitations, refers to the interference of non-creatinine chromogens with the Jaffe method. This was certainly a problem until about 20 years ago. There are, however, virtually no laboratories left that use this old method. At the very worst, many laboratories use a kinetic modification of the Jaffe method that eliminates most of the effects of protein and compounds such as ketone bodies. Most laboratories, and certainly all that use dry chemistry methods, do not use the Jaffe method at all, using an enzyme-based analytical procedure that is entirely free from the "interferents" that used to bedevil the old method. Only the effect of bilirubin (and to some extent haemoglobin), referred to above, remain as analytical challenges. A plot of serial reciprocal creatinine vs. time has been shown to be more accurate than taking a single urea or creatinine measurements to assess progress of renal disease. In most cases reciprocal serum-creatinine declines linearly with time as CRF progresses.
There is evidence that proteinuria is among the first (most sensitive) indicators of renal dysfunction. The average veterinarian's interpretation of and some of the literature relating to urine protein determination miss an essential issue. The urine dipstick, used almost exclusively as the veterinarian's first screening level for the identification of proteinuria, does not actually measure protein. Instead, it measures the urine albumin by a dye-binding method that is incapable of identifying most globulins and haemoglobin. Glomerular filtrate normally contains a small amount of albumin (due to its low mol. mass), almost all of this is reabsorbed by the proximal tubule cells in health. In the event that there is tubular dysfunction (loss of re-absorption), this albumin becomes detectable in the urine. When the glomerular membrane becomes disrupted, as in immune-complex-induced disease, the filtered load of albumin increases to the point that it exceeds the proximal tubule's ability to reabsorb. This then produces the proteinuria of glomerulopathy. The important issue is the fact that this proteinuria is principally an albuminuria. There is some evidence that urine protein electrophoresis can help in identifying the type of pathology present. The method is a little "fiddly" and difficult to standardise due to the variation in urine concentration.
Serum inorganic phosphate
Hyperphosphataemia is reported to be a late change in renal disease. Yet dogs with ARF that died had a much higher serum inorganic phosphorus (SIP) concentration than did dogs that survived. We have found a fairly strong correlation between elevated urea as well as creatinine and SIP. Our impression is that SIP is a fairly reliable indicator of renal dysfunction and it may even help in identifying acute pre-renal azotaemia from primary renal azotaemia. A lesser-known problem with SIP is the in-vitro instability of SIP and whole blood samples collected from veterinary practices by a once/twice-a-day courier service may yield falsely elevated SIP.
Urine concentrating ability
Urine specific gravity (SG) remains one of the easiest and most sensitive tests of renal function. There is evidence that loss of concentrating function is one of the earliest changes seen in renal dysfunction. Furthermore, SG data are critical in interpreting azotaemia. Routine refractometric estimation of the SG appears to make a significant contribution to the evaluation of renal disease cases at the author's institution although no studies have been conducted there to verify its utility.
Carbamylated haemoglobin CarHb)
Urea exists in equilibrium with cyanate/isocyanate which combines irreversibly with the terminal valine group of alpha and beta Hb chains to form CarHb. It accumulates throughout the 3 to 4-month life span of the erythrocyte in dogs with azotaemia. Its value reflects the average serum concentration of urea over that period. Patients that have had a short duration of azotaemia have the lowest CarHb concentration. Its concentration is significantly higher in dogs in CRF. The sensitivity and specificity for differentiating ARF from CRF of CarHb concentration was reported to be 96.1 and 84.2% respectively.
Urine enzyme measurement is a non invasive procedures used to assess renal tubular cell integrity. Many reports indicate that urinary enzymes (GGT and NAG) are a sensitive indicator of renal damage, in particular tubular lesions. Urine has disadvantages as a medium for quantitating enzyme activity. There is a high range of fluctuation in enzyme activity and some urine constituents (like urea) are fairly potent inhibitors of enzyme activity. A study in cats indicated that NAG and GGT provided the earliest indication of renal damage, correlating closely with renal pathology. Urine enzyme measurements do however use expensive reagents, are time consuming and are susceptible to variation due to a number of factors, not the least of which is variation in urine dilution. Urinary creatinine excretion for an individual is relatively constant, thus enzyme activities may be corrected for urine volume fluctuations if they are expressed as a factor of creatinine excreted.
Fractional excretion of electrolytes
Urine Electrolyte clearance ratios (Fractional excretion or FE) are calculated by comparing the clearance of an electrolyte with that of endogenous creatinine. Na is the most common electrolyte used. It is increased in tubular/renal failure but decreased in pre-renal azotaemia and normal or low in acute glomerular disease. There have only been 5 reports of the use of this parameter since 1999, three from the author's institution. The impression is that it is not likely that this test constitutes a major breakthrough for the average veterinary clinician.